The present invention is directed to an indication system and method for detecting and displaying the relative position between a tanker aircraft and a receiver aircraft during fuel transfer. The invention is intended to ensure a proper fuel transfer during the aerial refueling operations with Hose & Drogue systems.
In aerial refueling Hose & Drogue operations, the receiver pilot assumes the responsibility of conducting an engagement to enable fuel transfer, while maintaining a safe separation from the tanker. Contacts and disconnects are performed by the receiver aircraft following the indications from the refueling station. According to standard procedures, the tanker trails a hose having a free end that terminates in a reception coupling and a conical shaped drogue. Receiver aircraft are fitted with an aerial refueling probe which terminates in a fuel nozzle. Finally, the receiver aircraft is flown to engage the probe into the drogue.
This fact implies that the receiver aircraft pilot has the responsibility of getting the engagement and, after that, controlling and maintaining its relative position and speed with respect to the tanker aircraft all along the operation, from contact to disconnection. Fuel flow is started and held on to provided the engaged receiver maintains hose extension between certain maximum and minimum limits.
Currently, tanker aircraft refueling systems provide some helps for the receiver aircraft pilot to manage the operation, consisting basically of the following:                Signal or traffic lights.        Hose marks.        Tunnel lights.        
Regarding signal lights associated with each tanker aircraft, a set of rearward facing signal lights or display is installed, using colors red, amber and green. The lights provide indications of the operating status of the aerial refueling equipment, more specifically the refueling pod in this case. Following the standards, the color codes would be the following:                Red light, a red light means breakaway or do not make contact.        Amber light, amber light means clear contact.        Green light, a green light means fuel is flowing.        
All the known implementations of existing signaling lights for Hose & Drogue aerial refueling operations comply with this standard and therefore basically consist of a set of three visible colored lights or clusters and the corresponding infrared ones for covert mode.
Signal lights provide the receiver aircraft pilot with information on the status of the refueling station. Green light denotes that the receiver has pushed the drogue inside the refueling range. If the inner limit is exceeded, for instance because the receiver aircraft flies too close to the pod, the signal lights turn suddenly to flashing amber, requiring the pilot to move backwards and away and the pumps are immediately closed. If the outer limit is exceeded, for instance, when the receiver aircraft drops behind maximum refueling allowed hose extension, the signal lights turn suddenly to steady amber and the fuel pumps are closed immediately. In both cases, signal lights do not warn the pilot in advance.
Regarding hose dimension and markings, aerial refueling hoses include ring marks along certain intervals that aim to provide the receiver pilot with an indication of their deployment or retraction and their extended length during the contact as it is useful for the receiver pilot to be aware of the limits for the operation, such as the proximity to the minimum or maximum allowed hose extension, before disconnection.
Most hoses are marked with colored bands, mainly with a series of bands or a block of color to indicate the optimum receiver refueling position. On some hoses, the refueling position marks are bounded by additional markings indicating the start and stop positions for fuel flow. Usually, there is a series of closely spaced bands at the tanker end of the hose, these provide cues for the receiver pilot to assess rates of fore and aft movement after making contact or during disconnection.
Hose marking should be visible, as required by aircrew, when conducting aerial refueling operations in ambient lighting, weather and background conditions. The hose markings should provide the aircrew with visual cues relating to hose position, hose behavior, and receiver relative motion. The markings should also indicate the inner and outer limits of the refueling range and hose orientation aft of the inner refueling limit.
Hose marks are intended to provide a reference of the hose extension Inner and outer limit markings are entrusted to be clearly seen by the receiver aircraft pilot under any ambient lighting, weather and background conditions, but the fact is that it is quite hard for him to distinguish them, especially under night conditions. Several experiences confirm that the hose marks did not provide satisfactory cues for operation, as they were not visible when in contact and so could not be used for positioning, under twilight and night conditions.
Regarding tunnel lights it has to be taken into account that during night time conditions, the hose exit is illuminated from the refueling station rear part by visible lights, also known as tunnel lights. The drogue tunnel or the serving carriage of most tanker aerial refueling installations is lit from within. This is particularly useful for gauging the amount of hose pushed back into the hose drum. Additionally, the tunnel lights are intended to help view the marks on the hose when getting out of the refueling station rear fairing.
Tunnel lights are not fulfilling the function of illuminating the hose exit as expected. Therefore, at night or in degraded visual conditions, once contact is achieved, it is very hard to have a reference of the drogue position inside the refueling range, as the hose marks are nearly invisible. Refueling lights are not able to provide a precise indication of the whereabouts of the receiver aircraft inside the range, or in other words, how far away with respect to the inner or outer limit the receiver aircraft is. Tunnel lights illuminate only one small part of the hose close to the hose exit. As only a small part of the hose is illuminated, the pilot easily loses track of his position as he does not see the entire length of the hose. He is able to see marks close to the hose exit which do not give him a reference of the distance with respect to the limits.
As mentioned before, fuel flow is started and held on to provided the engaged receiver maintains hose extension inside certain maximum and minimum limits. As soon as any of those limits is exceeded, fuel flow is interrupted, and not resumed until the receiver gets back into the refueling range again and the pumps are reopened. These interruptions usually catch the pilot unaware, as the transition from fuel dispensing range to pumps closed is abrupt, and the above described help means are not totally effective to this purpose. These interruptions are extending the times to accomplish the aerial refueling operations and increasing the amount of workload required from receiver pilots.